*The below assumes a 1raxFE into Hellion / Banshee. If you open with 13 gas or multiple rax after FE, none of these busts should be very scary so long as you get 3ish Bunkers up.*
Scout to see if he has gas before 4 minutes. If he does, something early is likely coming... Banelings, Roaches, Speedling Expand, etc. If you think it's Banelings or Roaches, keep pumping Marines, build three bunkers in a triangle (2 in front, 1 in rear), covering each other, but not touching (roughly one bunker-width between each one). Try to get at least 2 Marines in each Bunker, and if you're really good, evacuate any Bunkers about to be hit by Banes. Have either an SCV or Hellion out on the field and keep it alive. Often, if he sees your defenses, he'll abort and go for a 3rd. You need to know that, so that you can salvage 2 of the Bunkers and try to take your 3rd.
If it's Banelings, he'll lose them all on your natural wall and Bunkers. Make sure the wall to your main is up and try not to lose any Hellions you have. Once the Banes are dead, get some SCVs to repair any surviving Bunkers. You need at least one Bunker and/or good Hellion control to survive a ling follow up. The MOMENT that you repel the attack, get 3 Bunkers (total) back up and then worry about any depot that you lost. He might try to bust you again. Again, make sure you scout for a 3rd. The moment he takes a 3rd, his ability to bust goes way down and he's going to go for a longer game.
If it's Roaches, you want to have SCVs on repair duty ASAP. If the Bunkers are behind a depot wall, but touching it, it's even better, because you can position the SCVs all the way in the back and he can't target them until he takes down the depots. You may not be able to do this on all maps, though. Rush that Banshee, if that's what you were teching to. Once it pops, his Roaches are on a timer and you can lift, run SCVs, etc until the Roaches are cleaned up.